Natural gas obtained from gas fields is liquefied in a liquefaction plant so that the gas may be stored and transported in liquid form; that is, as an LNG. Cooled to around −162 degrees Celsius, the liquid natural gas advantageously has a significantly reduced volume as compared to gaseous natural gas, and is not required to be stored under a high pressure. Generally, a natural gas liquefaction process involves removing impurities such as moisture, acidic gas components and mercury contained in raw material gas (natural gas to be liquefied) in advance as necessary, and further involves, after removing heavy components (cyclohexane, benzene, toluene, xylene or the like) having a relatively high freezing point in order to prevent clogging of piping, facility and the like, liquefying the raw material gas comprised primarily of methane. Methods for cooling the raw material gas include what is called “Propane pre-cooled Mixed Refrigerant Method” which includes pre-cooling the raw material gas with propane refrigerant, and cooling (liquefying) the gas with mixed refrigerant (nitrogen, methane, ethane, propane and the like).
This type of liquefaction system is usually provided as a plant with comparatively high throughput and requires a relatively large installation area. JP2000-180048A (Patent Document 1) discloses a liquefaction system in which raw material gas is cooled without propane pre-cooling, thereby eliminating facilities related to propane refrigerant to minimize an installation area of the liquefaction system.